McConnell issued a statement on Wednesday, saying: "John McCain is a hero to our Conference and a hero to our country.

"He has never shied from a fight and I know that he will face this challenge with the same extraordinary courage that has characterised his life.

"The entire Senate family's prayers are with John, Cindy and his family, his staff, and the people of Arizona he represents so well.

"We all look forward to seeing this American hero again soon."

McCain's office announced on Saturday that he would be away from the Senate the whole of this week.

McCain, a prisoner of war in Vietnam and a two-time presidential candidate, is known for his unfiltered opinions and willingness to buck Republican party orthodoxy.

Along with Senator Lindsey Graham, perhaps his closest friend in the Senate, McCain has become one of the leading Republican critics of President Trump, particularly on issues of foreign policy and national security.

In a written statement she posted on Twitter, McCain's daughter, Meghan McCain, said the news of her father's illness has "affected every one of us in the McCain family." She said they live with "anxiety about what comes next," which they have endured before. McCain has a history of melanoma, a dangerous form of skin cancer.

She added, "it won't surprise you to learn that in all this, the one of us who is most confident and calm is my father. He is the toughest person I know. The cruelest enemy could not break him."

US politicians from across the political spectrum were quick to praise McCain and wish him well.

President Donald Trump issued a statement saying he and first lady Melania Trump are sending "our thoughts and prayers to Senator McCain, Cindy, and their entire family."